A need exists for long wave infrared radiation (LWIR) detectors capable of performing when exposed to ionizing radiation. Such applications are found in space borne surveillance systems. Development of such detectors requires application of testing methods to evaluate detector performance in an ionizing radiation environment. This type of detector must be performance tested with an apparatus capable of shielding the detector from room temperature infrared radiation which would overwhelm the detector.
Presently, testing LWIR detector response to ionizing radiation within a low background infrared radiation environment is generally done by exposing detectors housed in cryogenic chambers to cobalt-60 gamma radiation. The gamma rays produce high-energy Compton electrons which penetrate the test sample and produce ionizing effects. The difficulty with this approach is that Compton electrons penetrate the test chamber with highly variable energies and trajectories due to random origin and scattering in the chamber. This causes considerable uncertainty in the interpretation of experimental results.
The detectors can also be tested by exposing them to energetic electrons from electron beam sources such as a linear accelerator or a scanning electron microscope as described by Flesner, L. D., et al., in "Electron-Beam Apparatus For Testing LWIR Detectors In A Cryogenically Shielded Environment", IEEE Transactions On Nuclear Science, Vol. NS-34, No. 6, December 1987. However, electron beams do not completely simulate the environment in which these detectors are to operate and have inherent limitations discussed below.
Other testing methods utilize X-rays. Ionizing radiation response testing using an X-ray source has the advantage over Co-60 radiation in that the X-ray can be selectively energized. An advantage of X-rays over energetic electrons is that the X-rays provide greater sample penetration for a given energy level. For example, 20 keV X-rays will penetrate with little attenuation through a 250 micrometer thick silicon wafer while electrons require an energy level greater than 200 keV to attain comparable penetration.
Conventional methods for X-ray testing involve propagating X-rays from a source which is external to the cryogenic test chamber through a window into the chamber. Since the X-ray source location is at a significant distance from the detector, this method requires X-rays of high intensity to compensate for divergence as they propagate from their source to the LWIR detector. There are numerous disadvantages to using high-intensity X-ray sources for this type of testing which include: expense, difficulty of modulation, and hazardous operation.
Therefore, a need exists for an apparatus which can test LWIR detector response to X-ray exposure with low intensity X-ray radiation within a low infrared radiation environment.